


What to Expect when...

by Astoria Gracewell (arh581958)



Series: What to Expect (an ABO series) [1]
Category: Mortal Instruments Series - Cassandra Clare, Shadowhunter Chronicles - All Media Types, Shadowhunter Chronicles - Cassandra Clare, Shadowhunters (TV)
Genre: Alpha!Magnus, Alpha/Beta/Omega Dynamics, Domestic Fluff, Family Fluff, Fluff, Fluff and Smut, M/M, Mpreg, Omega!Alec, Pre-Established Relationship, Pregnant Sex, RST, Romantic Fluff, Sex Games!, Smut, Tooth-Rotting Fluff
Language: English
Status: Completed
Published: 2016-03-07
Updated: 2016-03-07
Packaged: 2018-05-25 03:09:55
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,410
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/6177835
Author URL: https://archiveofourown.org/users/arh581958/pseuds/Astoria%20Gracewell
Summary: <blockquote class="userstuff">
              <p>... your mate is about five months along in his pregnancy. That, of course, means that Alec's extra-horny, extra-daring, and extra-wet. No one will ever excuse Magnus of not being a good mate. </p><p>(Or: Pregnant Sex in A/B/O verse because reasons)</p>
            </blockquote>





	What to Expect when...

**Author's Note:**

> **Warnings:** This story contains _MPREG_ in the A/B/O universe. If you are uncomfortable with the theme, I respectfully suggest that you turn back now because this will sear through your eyeballs and melt your brain into mush. I do not have the money to be held responsible. Thank you.
> 
> Beta-Read by the awesome Nerdling_Queen and Sairyn!

Magnus walked into their recently renovated three-bedroom apartment. By ‘recently renovated’, he meant that he had summoned a couple of pre-fabricated wood walls and doors from some unknown Home Depot, and made What’s-His-Face and the Daylighter do all the painting. He had after all done all the ‘heavy lifting’ which was snapping his fingers and making the materials magically appear out of thin air. 

While the pair grumbled and complained but ultimately did a far better job than he ever could. Clary and Jocelyn had done all the paintwork which why Alec and he were the proud owners of a periwinkle themed nursery, a purple and blue masters bedroom, and a forest green guestroom. 

Isabelle had volunteered to feed the poor, poor workers much to their dismay. She was a supporting aunt-to-be and brought handmade sandwiches for the entire week. Simon had taken one bite of the first sandwich and squished his face together. The brazen lad had eaten and entire sub and was sick for the remainder of the day. 

Now, the studio had been transformed into a fully functional family apartment. Even Maryse had dropped by on the fifth day to ‘baby-proof’ the entire apartment—the entire apartment: bathroom, den, living room, kitchen, patio,  _ everything _ . Not a surface was left untouched by her safety foam on the edges, not a cabinet was not child-locked, and not a shelf was left without a glass cover. She was very thorough. 

It wasn’t only the Nephilim who were supportive. Catarina came by over the course of the week to strengthen the protective spells over the apartment walls, paint sigils in her special paints, and added an extra-layer of tumble-proofing to all shaking surfaces. She also enchanted the nursery to have a special ‘warlock alarm’ that alerted the parents-to-be if any suspicious aura entered the baby room. 

With everyone pitching a little to help, the entire apartment was a melting pot of smells that Magnus now recognized as ‘family’ inside his head. It was a strange notion for him after years of living a bachelor’s life, after years of running away from commitment, after years of being afraid—Alec Lightwood came and gave him everything that he thought he’d given up. 

“Alec?” Magnus called out, scenting the air. Above the mingled scents, he found the one scent that had brought back light into his stagnant life.

“In the bedroom,” he heard from the hall. 

The entire layout of the apartment had changed when they renovated. Instead of an open-layout, they decided on sectioning portions of the space to give them and their offspring privacy from visitors. A section near the door was also sectioned-off to be Magnus’ ‘receiving area’ for his clients. In a few months, they will have a tiny half-warlock running around their home. 

He followed Alec’s scent almost in a haze. In all his life, he never smelled anything as enticing as Alec’s scent: a mix of rain, leather, and lilies. Of course, as Shadowhunters, they were on scent-blockers during their first encounter, and he’d only realized that the blue-eyed raven-haired Nephilim was an omega when Alec took a plunge into the freezing lake water. 

Alec had bravely, and somewhat stupidly, shared his energy with his (then secret) boyfriend. It had been a bold move that saved the ship from other demonic attack. It also was a very intimate experience. In his whole life, Magnus had only participated in energy-sharing twice. The second time had affected him more than he initially let on. It had opened a connection between them, stronger than anything either had ever encountered.

Now, that bond was fully consummated through traditional reciprocal mating-bites despite them being a non-traditional couple; both Alpha and Omega had bites on unorthodox places on their bodies.

“Hey there, gorgeous,” Magnus greeted from the door. He leaned an elbow against the door frame, taking in the sight of his mate, his lover, his children’s carrying father, curled up on the bed. The blankets were arranged in a strange oval shape with pillows, and blankets, and a whole lot of old clothes from everyone in their lives—themselves, Isabelle, Jace, Maryse, Clary, Simon, and Catarina. 

“Hey, babe,” Alec responded without moving from his position. He curled into the small ball of fur next to his face, the sleeping Chairman Meow. “Welcome home.” Only then did he move, shifting into a sitting position and causing the blanket to fall to his lap. 

“Oh,” Magnus said in realization, a bit winded. “You’re starting to nest, aren’t you?”

Alec appeared equally shocked at the statement. “What?” 

“I will bet a naming right veto powers that you’re fully naked under those sheets, Alexander,” Magnus purred as he walked closer. He first touched Alec’s calf over the thin canary sheets, feeling the omega shiver at the contact, then carefully smoothed his palm over the long, shapely legs which frequently wrapped around his waist as they made love. Then, with a tug, he stripped the barrier away. 

“M—Magnus!” Alec yelped in surprise. True enough, he  _ was _ completely bare underneath the thin layer of cotton. “It’s cold!” 

“Apologies, my love,” Magnus cooed, fingers levitating the cover back into place. “How are you?” he asked, leaning over to kiss his omega on the forehead. “Tough day?”

In reply, Alec glared. “Your  _ kid _ has been kicking me in the bladder the entire day. I barely stood a chance,” he declared in annoyance. “How am I supposed to go through my research materials if I can’t focus stay in bed long enough for a whole text!” 

When Magnus thought that his mate could not be any more endearing, he smiled at Alec’s small, lopsided pout that made his heart swoon all over again. “ _ Our  _ kid. But, I’m sorry.” He apologized sincerely. He knew how much it affected Alec to be out of patrol rotation, and even more so since he was ordered bed rest to keep from stressing out the baby. 

“It’s fine,” Alec shrugged. “But I deserve a prize for following Catarina’s orders.” 

“Oh, really?” One of Magnus’ eyebrows rose in interest. He crawled into bed on all fours, looking very much like a prowling cat in the Savanna. His eyes glinted to display his inner mischievousness that always seemed to come out around his mate. “And what prize would that be, my dear Alexander?”

“I can think of something,” Alec said, meeting Magnus’ grin tooth-for-tooth, “I was thinking of a  _ game _ .” He cooed, deliberately breathing hot air beside Magnus’ ear. Much like his feline companion, it was an erogenous zone for the warlock. A full-body shiver rocked through his frame. 

“Aren’t you too heavy for a chase, my love?” Magnus asked, voice gravelly in his arousal. Yet, his instinctive alpha sex-drive was overruled by the concern for his mate and their child. “Will you be alright?” 

“I’m no delicate flower, Bane,” Alec growled into his ear. An instant later, the ferocity was gone as Alec started to lick the shell of Magnus’ ear. “But I wasn’t talking about a chase.” 

“What do you have in mind?” Magnus inquired, pulling back. The scent of Alec, bare and willing, was overwhelming to his sensitive nose. And Alec knew, full well, his power over the warlock in this state. He could make Magnus do anything and the alpha would eagerly comply—anything to please his mate. Every day between them was a trial on trust. 

“I was thinking of a little game called—” In a display of his Shadowhunter strength, rare as it was given his pregnant state, Alec flipped them over, blankets tangling and all, until Magnus was lying down with his back pressed against the mattress. The alpha’s eyes and nostrils flared in arousal at his mate’s power. “—control.” 

“Oh, yes, Alexander,” Magnus writhed, “Manhandle me some more! You know how that gets me all hot and bothered.” 

“Shhh,” Alec silenced him and a finger to his lips. “I haven’t even told you the rules yet!” 

Magnus’ eyebrows shot up. “Ohhhh, rules, eh?” He licked his lips in anticipation. “Go on, tell me these  _ rules _ . You know I like coloring outside the lines,  _ Alexander _ .”

Alec tutted. “No,” he said with utmost seriousness, “I’m in control tonight. You play dirty and I’m sleeping in the guest room. You know how much I don’t like sleeping in a scentless place. Are you going to play along?” He ground his naked ass against Magnus’ clothed knot to make a point. 

“Yes!” Magnus hissed, grinding up at the contact, “Yes, yes, anything for you, Alexander.” 

“Good,” Alec grinned, kissing his lips in as a tiny consolation. He had every intention of making Magnus utterly come apart underneath his hands tonight and he was going to use every trick in his book to do it.

“You can touch me.” He said, grabbing Magnus’ hands and intertwining their fingers. The alpha mewled gratefully to be allowed to touch. Alec smirked. “But only in the places that I tell you to, okay?” He waited for Magnus to nod before beginning. “Magnus?” 

“O—okay,” Magnus sounded breathless beneath him. He loved how untraditional and non-posturing his alpha was when they were alone. It reminded him that they were  _ equals _ in their relationship and he loved Magnus more because of it. 

“Good alpha,” Alec praised, rubbing and scenting his cheek against Magnus’. 

“Face,” he said, bringing Magnus’ large caramel colored hands to his face. He mirrored the action with his own hands to Magnus’ face. “Follow me,” he instructed. He touched Magnus’s forehead first, tracing where the hairline ended, down the side, then dipping under the jaw. Then, he stroked over those meticulously kept eyebrows that would put Hollywood stars to shame. He loved how much the warlock made an effort in his appearance even if he, himself, completely lacked fashion sense.

Magnus’ breath hitched when Alec rubbed his earlobes, playing gently with the large black stud earing.

“You’re beautiful,” Alec whispered, voice floaty like he was in his own trance. 

“Hey,” Magnus called out, stroking Alec’s cheek, “Come back to me, Alec. We’re still playing a game.” 

Alec flushed in embarrassment, “Oh, uhm, neck,” he said, purring as Magnus complied. “Yes, just like that.” Magnus relished in feeling the warm, sweet-smelling, flesh of Alec’s skin beneath his fingers. He’d touched it so many times before but it always made pesky little butterflies erupt from his stomach to his chest. 

“Oh, Alexander,” he moaned, thumb sliding over Alec’s collarbone. “So responsive for me.” 

“Y—yeah,” Alec shivered, “Only for you, Magnus.”

“Where else, my love?” 

“Back,” Alec groaned, legs settling beside Magnus’ thighs. He settled like a large cat on Magnus’ chest, a reversal of their normal sleeping position pre-pregnancy, and buried his face in the crook of his mate’s neck, where the scent was the most pungent. “I want to feel your palms over my skin.”

Magnus obeyed. He leisurely ran his palms over the expanse of Alec’s back, from the edge of his spine to the curve of his ass, fingers tracing over the slightly raised scars, the pale silver scars, and the few runes that were permanently dark. Alec’s back was a canvas, a statement, of all his experiences as a Shadowhunter. He wore them all with pride. 

“You are wonderful, my Alexander, my  _ Omega _ .” 

Alec pressed closer at the title. A fresh wave of slick dribbled down his thighs. An omega, male or female, was able to produce their own natural lubricant to aid in the knotting process. Some omegas were still able to produce at a heightened state of arousal even when pregnant. Alec was always wet for his alpha, sometimes at the most inconvenient times! 

“You smell like sin,” Magnus grunted, hands trailing to the curve of Alec’s ass, only to be stopped by a soft plea. 

“Don’t.” Alec shook his head. “Not yet, babe, later.”

Magnus nodded in understanding. Alec often craved the slow build during their love making sessions. It was only in the throes of heat did he demand fast, unrelenting sex. Magnus was always eager to please his omega mate. “Tell me what you need, Alexander.”

“I—” Alec let out a broken whine as he ground their hips together, “Skin. Let me feel you. I want to feel your skin, alpha, rubbing against mine.” With a snap of fingers, his wish was granted. He keened as the heat of Magnus’ flesh ignited his. “Oh angels, Magnus, angels, holy angels.” 

Magnus chuckled. “I doubt they will appreciate you calling upon them when we tie, my sweet.” Alec’s cheeks flushed a light pink. “Hush, love, I was merely teasing. Are we still playing?” 

Alec’s snorted against his neck. “Of course we’re still playing.” He said definitively. He canted his hips and ground their erections together brazenly. He had come a long way from being the shy, closeted omega when they first met. Alec embraced his sexuality during their courtship and Magnus could not be any prouder. 

“You cheeky, cheeky, Nephilim!” Magnus giggled as Alec nipped across his jaw. “Don’t give me stubble burn!”

“Such a vain warlock,” Alec teased, rubbing his stubbly jaw across Magnus’ face. “You love it when you smell of me. Stop complaining,  _ mate _ .”

“Now that’s unfair,” Magnus complained, nosing Alec’s cheek. He rested his hands delicately over the curve of his mate’s round bum. “You’re using the m-word to manipulate me.”

“I wouldn’t say manipulate, per se.” Alec said coyly, with a blink of his long eyelashes for dramatic effect. It wasn’t a surprise that the warlock’s flare for theatrics had somehow gotten into his system. “Think of it as a highly effective mode of persuasion.”

Magnus laughed, full-bodied and rumbling down to his gut. “Okay, fine, if you insist, my love… now, where do you want me touching you next?” 

“I want you to lick me,” Alec said bluntly. “Until I smell so much of you that no one will even think that I’m unmated—” he leaned in, aligning their bodies together from shoulder to waist. “I don’t want anyone doubting that I am your mate and that it’s your pup what I am carrying.” 

That sentiment made Magnus’ inner alpha flare with possessiveness, his scent exploding from his pores and washing over Alec. His mate keened and whimpered as the scent invaded in senses, filled his heart with love and adoration. Alec wanted to bury himself in Magnus’ scent, under Magnus’ skin until his mate was ingrained in every cell of his body. 

“My omega, my mate, my love,” Magnus growled, “my Alexander”

To which, Alec simply replied with “Yours.” 

Magnus nipped, and licked, and suckled, every inch of flesh which he could reach while his precious mate was on top of him. He scented the all-organic shampoo in Alec’s hair, nuzzled the skin under Alec’s ear, and tasted the sweetness of Alec’s lips. His senses ran on overdrive, pulling him into the alcove of Alec’s body. 

He whispered words of endearment. I-love-you-s, I-need-you-s, and I-want-you-s slurred together in an incoherent chant while his mind raged with  _ matelovecherishprotectworshipAlec _ over and over again. He ran his finger over the dip of Alec’s spine, nostrils scenting the air fresh with Alec’s slick while his mate rubbed his leaking erection between their bodies. 

“Yes,” Magnus chanted while his hands guided Alec’s arrhythmic rutting. “That’s it, that’s it, Alexander, take your pleasure, my love.”

“Magnus,” Alec whispered reverently like his name was a prayer. “Magnus, ‘is not enough. You need… you need to touch me… I need you… I need you inside… filling me with seed.”

Magnus was so hard, his know threatened to knot at any moment. Not only was his omega wet, willing, and wanting above him, grinding against his cock in reckless abandon but Alec’s words were so filthy that it sounded like he was in—

“Heat,” he realized with a shock, “Alec, you’re in heat.”

“Y—yeah,” Alec breathed out, “I—I read it in the  _ What to Expect _ books. Preg—ugh, Magnus, I need you now, please, please finger me, please, babe, please—pregnancy heat.” He let out an ear-piercing yell which made Magnus thankful for their added privacy spells that blocked noise from scandalizing their mundane neighbors. 

“Damn, Alec, I love it when you’re vocal.” Magnus chuckled, running his hands appreciatively over the curve of Alec’s hips. “Let me hear you, my precious Nephilim, let me hear you scream my name.” 

“ANGELS!” Alec screamed when Magnus breached him with two fingers, straight to the second knuckles. “Dear Angels, Magnus, Magnus, please, s’good, feel so good… more, give me more, I want your knot plugging me up while you fill my womb over and over again.”

Magnus was only alpha. He leveraged his legs, bending at the knee and planting both feet on the bed so that Alec can use it as a makeshift back rest when they tied. He coaxed his omega to sit on his lap instead of sprawled on his chest, making Alec whine at the loss of body heat. And all the heavenly angels, Alec was beautiful like this—plump and pregnant above him.

“Relax, love,” he reassured, “It’ll be better soon, I promise.” 

“Knot me,” Alec started to beg, “Knot me please, Magnus.”

“Lift up,” Magnus commanded with his alpha-voice, the only way which Alec would be able to obey in his state of arousal. His omega obediently followed, rising shakily to his knees to straddle the alpha. “Good omega, you’re fabulous Alexander.” 

“Hurry up and knot me, useless alpha!” Alec growled as he wiggles on Magnus’ legs.

In one powerful thrust, Magnus sheathed himself inside his omega, making Alec howl like an animal. He set up a torturously slow pace, lifting Alec by the hips before bringing him down to take every inch of his nine-inch long alpha cock. He repeated the motion, slow and steadily, until they were both beaded in sweat, and vibrating with need. 

“Faster,” Alec demanded. 

“No,” Magnus shook his head. “I’m not endangering the baby. You will come like this—untouched if you have to.” 

Alec whined brokenly at the back of his throat, sobbing. His rim was sensitive from the pregnancy and every glorious inch of Magnus cock sliding in and out of him was pure pleasurable torment. He nearly teared-up when Magnus refused to change their leisurely pace. He wanted—he needed—but he trusted his alpha to know what’s best.

The intensity behind his orgasm crept up from nowhere, catching him off guard. He spurted robes of white omega milk, painting his alpha’s chest in thick streaks. Caught in his post-orgasm haze, he lazily ran his hands over the stain spreading it across Magnus’ warm dark skin, reveling in how much he loved seeing his pleasure marking his alpha’s skin. 

“I love you,” he told Magnus sincerely, and the alpha erupted like a dormant mountain inside him. The knot inflated, locking them together, and Alec was stretched to the brim, so full, and so sated. There was no feeling in the world that could compare to the intimacy of a tying. He reveled in it, basked in it, relished in the biological proof that they were made for each other. 

“Hey there, Alec,” Magnus whispered to him. It could be seconds, minutes, or hours later. He felt like his body was in a whole ‘nother dimension where time and space were irrelevant—until he crawled his way back from the abyss, allowing Magnus’ voice to guide him. “Hey, my love, my sweet, sweet omega. Welcome back.” 

“Hi,” Alec replied shyly, “How long—” Oh angels, he said inwardly, feeling the rasp in his voice. “—how long was I out?” 

“Half an hour,” Magnus responded with a chuckled. “I think a few minutes and I can pull out.” It was only then that Alec recognized the ocean-like waves across his backs were actually Magnus’ hands smoothing over his spine in a tender motion. 

“Don’t,” Alec caught himself saying and instantly understanding was in Magnus’ gold-green eyes. 

“I love you, Alexander Lightwood.” Magnus said with pure sincerity. He kissed Alec on the nose before he readjusted their position. He made it so most of the strain was on his body rather than his mate’s. He did not want Alec feeling more cramped muscles than necessary. “If we get untied, I promise to knot you again in a better position.” 

By then, adrenaline and the post-coital high was receding and the exhaustion was starting to kick in. Alec hummed a sleepy response: “Sounds good.”

**Author's Note:**

>  **THIS IS A MILESTONE FOR ME!** It's my 100th story as a writer on Ao3 _and_ my 30th story in the TMI/Shadowhunters Fandom! *crieswithpurejoy* So, to celebrate, I decided to go a little bit out of my normal style and present you with playful banter, shameless groping, and (wait for it) heat!sex. BAM! I hope you enjoyed it!
> 
> If you have a prompt or an idea, you can [**INSPIRE ME**](http://arh581958.tumblr.com/submit) on tumblr.
> 
> P.S. Can you be extra-nice? **kudos/comment/bookmark or _do all three_ **if you like the story! :)****


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